1. Field of the Invention
This invention relates to grid-modulated plasma switches, generally referred to as CROSSATRON.RTM. switches, and to the operation of such switches at current levels of 10kA or greater.
2. Description of the Related Art
CROSSATRON switches are grid-modulated plasma switches capable of fast closing speeds like a thyratron, and of rapid opening like a vacuum tube. CROSSATRON is a registered trademark of Hughes Aircraft Company. A sequence of CROSSATRON designs are shown in U.S. Pat. No. 4,247,804 issued Jan. 27, 1981 to Harvey, U.S. Pat. No. 4,596,945 issued Jun. 24, 1986 to Schumacher et al. and U.S. Pat. No. 5,019,752 issued May 28, 1991 to Schumacher, all of which are assigned to Hughes Aircraft Company, the assignee of the present invention.
The principles of operation of a CROSSATRON switch are illustrated in FIG. 1. The switch is a hydrogen plasma device having four coaxial, cylindrical electrodes disposed around a center axis 2. The outermost electrode 4 is the cathode, which is surrounded by an axially periodic permanent magnet stack 6 to produce a localized, cusp magnetic field 8 near the cathode surface. The innermost electrode 10 functions as an anode, while the next outer electrode 12 is a control grid and the third outer electrode 14 is a source grid.
Secondary electrons produced at the cathode surface are trapped in the magnetic field, and travel in cycloidal E.times.B orbits (where E is the electric field and B is the magnetic field) around the cylindrical anode 10 due to the radial electric field and the axial component of the magnetic field. The electrons eventually lose their energy via collisions, and are collected by the anode or grids. The long path length of the electrons near the cathode surface enhances ionization of the hydrogen background gas, and reduces the pressure at which the switch operates (compared to thyratrons). The hydrogen pressure in the switch can range from 100 to 1,000 microns, depending upon the gap spacing between the electrodes and the voltage level. The cathode material is typically molybdenum, and no cathode heater power is required.
The source grid 14 is used to minimize turn-on jitter by maintaining a low level (typically less than 20mA) DC discharge to the cathode, while the control grid 12 is normally held within about 1kV of the cathode potential. When open, the high voltage in the switch is sustained across the gap between the control grid 12 and the anode 10. The switch is closed by pulsing the control grid to a voltage potential above that of the cathode, thereby building up the density of the plasma 16 so that it diffuses into the gap between the control grid 12 and the anode 10. The result is a low impedance conduction path between the cathode and anode, and a consequent closing of the switch. A high density plasma can be established in the switch, and the rate of current rise to the anode can be increased by prepulsing the source grid 14 at about 1 microsecond before the closing voltage pulse is applied to the control grid 12.
The CROSSATRON switch was originally developed as a closing-only switch (U.S. Pat. No. 4,247,804), but a modulator switch capable of high current interruption was also developed (U.S. Pat. No. 4,596,945). In U.S. Pat. No. 5,019,752 the cathode was provided with a series of chromium-plated circular grooves or corrugations that extended around the cathode axis. The corrugations increased the effective cathode surface area exposed to the plasma, and thereby reduced the electron emission current density from the chrome surface to minimize arcing.
A different approach to the use of cathode corrugations was disclosed in an application by the present inventors, "High Voltage Crossed-Field Plasma Switch" Ser. No. 07/901,353, filed Jun. 19, 1992 and assigned to Hughes Aircraft Company. The cathode corrugations in this application extend axially, rather than circumferentially as in the '752 patent, with the corrugation depths being at least twice their widths. When used in connection with a deuterium gas fill, switching voltages greater than 100kV and a peak closing current of 1kA were achieved, as compared with a peak closing current of about 250 amps with a more conventional flat cathode surface and hydrogen fill.
The current level achieved with the above switch was still not high enough to allow the switch to be used for laser discharge switching applications, such as those found in TE-CO.sub.2 and excimer lasers. These applications require the switch to have a peak current capability of about 2.5-10kA, and also a closing speed greater than 2.times.10.sup.10 A/sec for CO.sub.2 lasers and approximately 1.times.10.sup.11 A/sec for excimer lasers. At present, gas-discharge lasers utilize thyratrons, such I as described in Cobine, "Thyratron", McGraw-Hill Encyclopedia of Electronics and Computers, McGraw-Hill Inc., 1984, pages 855-856, and spark gaps. Since CROSSATRON switches have a much longer life than thyratron and spark-gap switches, plus similar fast closing speeds and much higher pulse-repetition-frequencies, it would be desirable to use CROSSATRON switches for gas laser systems. However, currently available CROSSATRON switches are limited to peak currents of 3kA or less. Attempts have been made to increase the peak current level by increasing the cathode diameter, and thus the electron-emitting area; switches with a peak current capability in excess of 10kA have been achieved by using cathode diameters in excess of 25 cm. Unfortunately, commercial lasers have a fixed diameter socket into which the switch must fit, and CROSSATRON switches with cathode diameters in excess of about 10 cm cannot be accommodated. Therefore, although the high current CROSSATRON switches that have been developed exhibit a peak current capability that is sufficient for laser switching, in practice they are much too large to be used for laser applications.